Biomimetic 2D transistor for audiomorphic computing

ABSTRACT

Embodiments relate to a computing device that may be configured as a biomimetic audiomorphic device. The device can include a field effect transistor (FET) having a split-gate architecture with different spacing between the split-gates. Embodiments of the device can include multiple split-gates. Some embodiments include the integration of delay elements and tunable resistor-capacitance (RC) circuits for imitating the interaural time delay neurons. Some embodiments include global back-gating structural features to provide neuroplasticity aspects so as to provide adaptation related changes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national stage application under 35 U.S.C. §371 for International Patent Application No. PCT/US2020/043871, filed onJul. 28, 2020, which is related to and claims the benefit of priority ofU.S. provisional application 62/880,983, filed Jul. 31, 2019, the entirecontents of each is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.FA9550-17-1-0018 awarded by the United States Air Force/AFOSR. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

Embodiments relate to a biomimetic audiomorphic device. The device caninclude a field effect transistor (FET) having a split-gatearchitecture. Embodiments of the device can include multiple split-gateswith different spacings between the split-gates of the split-gatearchitecture. Some embodiments include the integration of delay elementsand tunable resistor-capacitance (RC) circuits for imitating interauraltime delay neurons. Some embodiments include global back-gatingstructural features to provide neuroplasticity or reconfigurabilityaspects to the device.

BACKGROUND OF THE INVENTION

Supercomputers have become so powerful that they can easily surpass theperformance and capacity of the human brain in processing speed andamount of information storage.

However, there exists a gap in energy consumption and area efficiencybetween the human brain and the supercomputers with the supercomputersbeing at least 1000000× less efficient than the human brain. Theemerging era of neuromorphic computing promises to shrink this gap bydeploying artificial neural networks (ANNs). ANNs mimic the fundamentalcomputing unit of brain i.e. neurons connected to other neurons viasynapses. Neuromorphic chips, such as TrueNorth, Loihi, BrainScale andSpiNNaker are some examples of advancements in the area of artificialintelligence, however, these chips will still remain overwhelminglypower hungry when scaled up to the full capacity of human brain, whichhas ˜100 billion neurons connected via 1 quadrillion synapses operatingat a miniscule ˜20 W power.

BRIEF SUMMARY OF THE INVENTION

Embodiments relate to a biomimetic audiomorphic device. The device caninclude a field effect transistor (FET) having a split-gatearchitecture. Embodiments of the device can include multiple split-gateswith different spacings between the split-gates of the split-gatearchitecture. Some embodiments include the integration of delay elementsand tunable resistor-capacitance (RC) circuits for imitating interauraltime delay neurons. Some embodiments include global back-gatingstructural features to provide neuroplasticity or reconfigurabilityaspects to the device.

In an exemplary embodiment, the biomimetic audiomorphic device includes:a substrate to serve as a back-gate; an insulator or oxide layer formedon the substrate to serve as a back-gate dielectric; a semiconductingchannel formed on the back-gate dielectric layer; metallic layers formedin and/or on the semiconducting channel to define the source and thedrain contacts; an insulator or oxide layer to serve as a top-gatedielectric; a metallic layer formed on the top-gate dielectric to definethe split-gate pair, the split-gate pair comprising a first gate and asecond gate. The first gate is formed between the source and the drainlocated on a first side of the FET device; the second gate is formedbetween the source and the drain on a second side of the FET device; andthe first gate is physically separated from the second gate.

In some embodiments, the substrate is Si; however, any substrate can beused. In addition, the substrate can be flexible or rigid.

In some embodiments, the back-gate oxide layer is SiO₂; however, anyoxide can be used such as Al₂O₃, HfO₂, h-BN, etc.

In some embodiments, the semiconducting channel is MoS₂; however, anyorganic or inorganic semiconductor can be used, such as other 2Dmaterials, Si, oxide semiconductors, compound semiconductors, etc.

In some embodiments, the metallic layer is a stack of nickel or gold;however, any metal stack can be used. For instance, the source, drain,and gate contact can be metals, semi-metal, and heavily dopedsemiconducting materials.

In some embodiments, the top-gate oxide layer is Hydrogen silsesquioxane(HSQ); however, any oxide can be used such as Al₂O₃, HfO₂, h-BN, etc.

In some embodiments, the biomimetic audiomorphic device, when inoperation: the back-gate is biased at a voltage V_(BG), source isgrounded and drain is biased at a drain-to-source voltage V_(DS), thefirst split-gate is biased at a split-gate voltage V_(SG1), the secondsplit-gate is biased at a split-gate voltage V_(SG2), and asource-to-drain current I_(DS) flows through the channel.

In some embodiments, plurality of split-gate pairs are used.

Each split gate is physically separated from any other split gate.

In some embodiments, each split-gate pair is separated by a distance, d,and: the distance between the first split-gate pair and the secondsplit-gate pair is d₁; the distance between the second split-gate pairand the third split-gate pair is d₂; the distance between the thirdsplit-gate pair and the fourth split-gate pair is d₃; the distancebetween the fourth split-gate pair and the fifth split-gate pair is d₄;and d₁=d₂=d₃=d₄.

In some embodiments, d₁=400 nm, d₂=400 nm, d₃=400 nm, and d₄=400 nm.

However, any spacing, d, between the split-gate pairs can be used.

In some embodiments, each first gate and second gate of each split gatepair is separated by a distance, s, and: the distance between the firstgate and the second gate of the first split-gate pair is s₁; thedistance between the first gate and the second gate of the secondsplit-gate pair is s₂; the distance between the first gate and thesecond gate of the third split-gate pair is s₃; the distance between thefirst gate and the second gate of the fourth split-gate pair is s₄; thedistance between the first gate and the second gate of the fifthsplit-gate pair is s₅; and s₅>s₄, s₄>s₃, s₃>s₂, and s₂>s₁.

In some embodiments: s₁=200 nm, s₂=300 nm, s₃=400 nm, s₄=500 nm, ands₅=600 nm. However, any spacing, s, between the first gate and secondgate in the split-gate pair can be used.

In some embodiments, the biomimetic audiomorphic device further includesa plurality of fullytop-gated field effect transistor (FET) devices orresistors made out of any technology.

In an exemplary embodiment, a fullytop-gated FET device includes: asubstrate to serve as a back-gate; an insulator or oxide layer formed onthe substrate to serve as a back-gate dielectric; a semiconductingchannel formed on the back-gate dielectric layer; metallic layers formedin and/or on the semiconducting channel to define the source and thedrain contacts; an insulator or oxide layer to serve as a top-gatedielectric; metallic layer formed on the top-gate dielectric to definethe top-gate.

In some embodiments, the fully-top gated FET device, when in operation:the back-gate is biased at a voltage V_(BG), source is grounded anddrain is biased at a drain-to-source voltage V_(DS), the top-gate isbiased at a voltage V_(TG), and a source-to-drain current I_(DS) flowsthrough the channel.

In an exemplary embodiment, each individual split-gate is connected tothe drain of an individual fullytop-gated FET device.

Further features, aspects, objects, advantages, and possibleapplications of the present invention will become apparent from a studyof the exemplary embodiments and examples described below, incombination with the Figures, and the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

The above and other objects, aspects, features, advantages, and possibleapplications of the present invention will be more apparent from thefollowing more particular description thereof, presented in conjunctionwith the following drawings. It should be understood that like referencenumbers used in the drawings may identify like components.

FIG. 1 shows how the path difference between the sound waves reachingthe two ears is translated into interaural time difference (ITD) for anacoustic source located at an azimuth angle, θ, for a given head radius(r_(H)) and sound velocity (v_(S)).

FIG. 2 shows ITD as a function of θ for different head sizes.

FIG. 3 depicts the Jeffress model with three essential neuralcomponents: (1) the time delay neurons, (2) the coincidence detectorneurons, and (3) a spatial computational map that correlates the two.

FIG. 4 shows a simple anatomical drawing of barn owl's brainstem whichbears astonishing similarity with the Jeffress model.

FIG. 5 shows exemplary schematics of an embodiment of the biomimeticaudiomorphic device configured to simulate a solid-state coincidencedetector neuron and an embodiment of a tunable resistor-capacitor (RC)circuit configured to simulate a delay neuron.

FIG. 6 shows an exemplary schematic of an embodiment of a fullyintegrated biomimetic audiomorphic device in order to emulate the neuralcomputational map inside the auditory cortex of barn owl following theJeffress model.

FIG. 7 shows an exemplary schematic of a fully top-gated MoS₂ FET on aconventional Si substrate.

FIG. 8A shows the transfer characteristics, i.e. the source to draincurrent (I_(DS)) as a function of the top-gate voltage (V_(TG)) for asource to drain voltage of V_(DS)=1 V of an embodiment of the device ofFIG. 7 , and FIG. 8B shows a corresponding truth table.

FIG. 9 shows the schematic of a MoS₂ FET with a split-gate architecture.

FIG. 10A shows the transfer characteristics, i.e. the source to draincurrent (I_(DS)) as a function of the split-gate voltage (V_(SG)) for asource to drain voltage of V_(DS)=1 V of an embodiment of the device ofFIG. 9 , and FIG. 10B shows the corresponding truth table.

FIG. 11 shows the source to drain current (I_(DS)) versus time forrandom voltage pulses of amplitude −30 V applied to the split gates ofFIG. 9 .

FIG. 12 shows a COMSOL multiphysics simulation of a 2D potential profilewhen −30V bias is applied to either one or both split-gates of the FETof FIG. 9 .

FIG. 13 shows the 1D potential profile along the channel width fordifferent combinations of two split-gate biases.

FIG. 14 shows the simulated transfer characteristics of the split-gatedMoS₂ FET using the Virtual Source (VS) model and the electrostaticpotential profile, V_(CH) (X) along the channel width obtained from theCOMSOL simulations.

FIG. 15 shows the biomimetic audiomorphic device with a plurality ofsplit gates.

FIG. 16 shows the transfer characteristics of the device when each splitgate of the FIG. 15 are concurrently swept from 0V to −30V.

FIG. 17 shows the inhibition ratio (IR) color map for all possiblecombinations of the split-gates of the FIG. 15 .

FIG. 18 shows the COMSOL simulation results for the 1D potential profileacross the channel width for different split-gate spacing.

FIG. 19 shows the angular precision (Δθ) and number of analog levels(N_(P)) as a function of ΔIR_(D) for different values of I_(ON)/I_(OFF).

FIG. 20 shows the output current from an embodiment of the biomimeticaudiomorphic device when random voltage spikes of magnitude −30 V areapplied to two spilt gates corresponding to different spatial pairs.

FIG. 21 shows the schematic of an artificial time delay neuron realizedby connecting the gate of an embodiment of a split gate FET device to adrain of top-gated FET device.

FIG. 22 shows the experimental transient responses of an embodiment ofthe artificial time delay neuron.

FIG. 23 shows transfer characteristics of a top-gated FET device atV_(DS)=1V.

FIGS. 24A and 24B show an embodiment of the biomimetic audiomorphicdevice integrating the split-gate FET device architecture and an RCcircuit architecture.

FIG. 25 shows an exemplary schematic of a FET device with globalback-gating capability.

FIG. 26 shows the back-gate transfer characteristics for V_(DS)=1V.

FIG. 27 shows the transfer characteristics of an embodiment of thebiomimetic device when the split-gate pair are concurrently swept from0V to −30V under different back-gate (V_(BG)) biases.

FIG. 28 shows the inhibition ratio (IR) as a function of V_(BG) fordifferent split-gate spacing extracted from the corresponding transfercharacteristics.

FIG. 29 shows the back-gate transfer characteristics of a fullytop-gated MoS₂ FET for different top-gate voltages (V_(TG)) atV_(DS)=1V.

FIG. 30 shows the extracted IR from the VS model simulation results as afunction of V_(BG) for various split-gate spacing ranging from 50 nm to600 nm.

FIG. 31 shows the color map of IR for all possible split-gate pairs ofour biomimetic device under different back-gate biases.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of an embodiment presently contemplated forcarrying out the present invention. This description is not to be takenin a limiting sense but is made merely for the purpose of describing thegeneral principles and features of the present invention. The scope ofthe present invention should be determined with reference to the claims.

In developing the biomimetic audiomorphic device 104, as well as thecomponents and other aspects of the biomimetic audiomorphic device 104,the inventors used the auditory signal processing of a barn owl as amodel. The ability to localize sound is an essential survival featurefor predators and preys alike explaining why terrestrial vertebrateshave two ears so that the interaural time difference (ITD) of lowfrequency sound waves can be used as a cue to determine the direction ofits source. FIG. 1 shows how the path difference between the sound wavesreaching the two ears is translated into ITD for an acoustic sourcelocated at an azimuth angle, θ, for a given head radius (r_(H)) andsound velocity (v_(S)). FIG. 2 shows ITD as a function of θ fordifferent head sizes. Clearly, for typical size of animal heads it isimperative that the auditory information must be processed withinhundreds of microseconds using neurons which can fire only once per fewmilliseconds. Therefore, auditory signal processing is a challengingcomputational task for any animal. Remarkably, this problem has beenevolutionarily resolved using smart neural algorithms implementedthrough befitting neurobiological architectures. In this context, barnowls are model auditory systems owing to their extraordinary ability todetermine the location of sound with a precision of 1-2 degrees evenwhen hunting in total darkness. In 1948, Lloyd Jeffress published aseminal paper, where he formulated a model that describes how acoustictiming differences are represented as a “place” in an array of nervecells or in other words how the brain transforms temporal coding intospatial coding. Remarkably, the key assumptions of his model are valideven today. In fact, the Jeffress model remains as the cornerstone forthe neurophysiological understanding and development of mostcomputational models for sound localization.

FIG. 3 depicts the Jeffress model with three essential neuralcomponents: (1) the time delay neurons, (2) the coincidence detectorneurons, and (3) a spatial computational map that correlates the two.The coincidence neurons fire only when spikes arrive concurrently to thecorresponding delay neurons. The length of the delay neurons from theleft and right cochlear nuclei is equal up to the points X and Y.However, beyond these points these delay neurons are projected ontocoincidence neurons in a ladder-like branching structure that runs inopposite directions. The delay neurons from the right side form a set ofcollaterals so that the axonal length to the coincidence neuron islonger for cell 7 than for cell 1 and vice versa for the delay neuronsfrom the left side. Due to the finite speed of axonal conduction, thesebranching patterns constitute a map for the ITDs, and hence correlatesto the spatial location of the sound source. For example if a soundoriginates from straight ahead, it will reach the right and left cochleaat the same time, i.e. without any interaural delay (ITD=0 μs). In thiscase only neuron 4 will receive coincident inputs since the total axonalpath lengths are equal. However, if the sound originates from the lefthemisphere, it will reach the left ear earlier than the right ear. Inthis case, coincidence will occur if the input signal from the lefttravels a longer path length than that from the right by an amount thatexactly offsets the acoustic delay, e.g., at cell 1. Similarly, if thesound originates from the right hemisphere, coincidence will occur e.g.,at cell 7. Therefore, the spatial projection of the right and left delayneurons onto the coincidence neurons form a computational map where eachcoincidence neuron represents a specific ITD corresponding to anazimuth.

FIG. 4 shows a simple anatomical drawing of barn owl's brainstem whichbears astonishing similarity with the Jeffress model. In the barn owl,the axons of secondary nerve fibers from the nucleus magnocellularis(NM) provide the delay lines. These monaural channels originate fromleft and right cochlear nuclei and converge on binaural tertiary nervefibers which serve as coincidence detectors in the nucleus laminaris(NL). Neurons in NL only discharge on receiving coincident spikes fromtheir monaural, excitatory afferents.

FIG. 5 shows exemplary schematics of an embodiment of the biomimeticaudiomorphic device 104 configured to simulate a solid state coincidencedetector neuron and an embodiment of a tunable resistor-capacitor (RC)circuit 102 configured to simulate a delay neuron. As will be explainedin detail herein, embodiments of the biomimetic audiomorphic device 104can include a split-gated MoS₂ field effect transistor (FET)architecture used to achieve the functionality of coincidence detectorneuron, wherein the tunable resistor-capacitor (RC) circuit 102 is usedto mimic delay neurons.

FIG. 6 shows an exemplary schematic of an embodiment of a fullyintegrated biomimetic audiomorphic device 104 in order to emulate theneural computational map inside the auditory cortex of barn owlfollowing the Jeffress model. As can be seen from FIG. 6 , thebiomimetic audiomorphic device 104 includes multiple FET devices 100having multiple split-gates 106 with different widths of the ungatedregion on a single channel 108. The channel is connected to the source110 and drain 112 terminals of the biomimetic audiomorphic device 104 torealize the computational map. Each split-gate 106 is connected to adelay line resistor with the resistance value designed in accordancewith the spatial location of the corresponding split-gate 106. With sucha configuration, a single biomimetic audiomorphic device 104 can performbiomimetic audiomorphic computing by concurrently performing digital andanalog computations.

In the exemplary embodiments, MoS₂ is selected as the semiconductingmaterial for the FET device 100 channel 108 material. It should be notedthat selection of MoS₂ as the semiconducting channel material isexemplary only. It should be understood that embodiments of thebiomimetic audiomorphic device 104 need not be restricted to a MoS₂ FET,rather any semiconducting material that allows for the implementation ofthe split-gate FET geometry can be used.

It is relatively straightforward to recognize that a coincident neuronacts like a two input AND gate because it fires maximum number of outputspikes only when it receives signals from both the left and the rightdelay neurons at the same time. Coincidence circuits are known for theirability to greatly minimize the chance of a false detection. If theprobability of falsely identifying a noise pulse as a genuine signal byone detector is P, then the probability of false detection when twodetectors detect the signal pulse simultaneously is P². Therefore, ifP=0.1, then P²=0.01. Thus, the probability of false detection can besignificantly reduced by the use of coincidence detection.

FIG. 7 shows an exemplary schematic of a fully top-gated MoS₂ FET on aconventional Si substrate. The semiconducting MoS₂ channel is few atomiclayers thick and is connected to Ni/Au metal contacts that serve as thesource/drain terminals. 120 nm of hydrogen silsesquioxane (HSQ) is usedas the top-gate dielectric and Ni/Au is used as the top-gate electrode.

FIG. 8A shows the transfer characteristics of the device—e.g., source todrain current (I_(DS)) versus top-gate voltage (V_(TG)) for a drainbias, V_(DS)=1V. The device is normally ON at V_(TG)=0V due tounintentional n-type doping and metal Fermi level pinning close to theconduction band of MoS₂ and can be gradually turned OFF by applyingnegative V_(TG). A high current ON/OFF ratio of −10⁶ is achieved and thedevice can be treated as a one-input-one-output digital elementrepresented by the truth table shown in FIG. 8B. Evidently this devicecannot represent an AND logic.

On the contrary, FIG. 9 shows the schematic of a MoS₂ FET device 100.The FET device 100 includes a MoS₂ semiconducting material exfoliated onSi/SiO₂ gate stack. An oxide layer 116 can be formed in and/or on atleast a portion of the source 110 and at least a portion of the drain112 so as to span a length between the source 110 and the drain 112. Thesemiconducting region MoS₂ 108 which is underneath the oxide layer 116is the active region of operation and lies between source 110 and drain112. A metallic layer 118 can be formed on top of the oxide layer 116,wherein the metallic layer 118 and the oxide layer 116 form a split-gate106. The split-gate 106 includes a first gate 106 a formed between thesource 110, the drain 112, and a MoS₂ semiconducting material 108 on afirst side 122 of the FET device 100. The split-gate 106 includes asecond gate 106 b formed between the source 110, the drain 112, and aMoS₂ semiconducting channel material 108 on a second side 124 of the FETdevice 100. The first gate 106 a is physically separated from the secondgate 106 b. In this configuration, the source 110 may be connected toground so that in operation, the drain 112 is biased at a drain-sourcevoltage V_(DS), the first gate 106 a is biased at a split-gate voltageV_(SG1), the second gate 106 b is biased at a split-gate voltageV_(SG2), and a drain-source current I_(DS) flows through the channel108. The source 110 and drain 112 voltages are biased at 0 and 1Vrespectively and the current I_(DS) flows from drain 112 to source 110.The split gates V_(SG1) 106 a and V_(SG2) 106 b correspondingly modulatethe drain to source current when biases are applied.

FIG. 10A shows the transfer characteristics of the MoS₂ FET device100—e.g., source to drain current (I_(DS)) versus split-gate voltage(V_(SG)) for a drain bias, V_(DS)=1V under two different conditions. Onecurve depicts the MoS₂ FET device 100 characteristics when one of thesplit-gates 106 is swept from 0V to −30V, while the other split-gate 106is held at a constant bias of 0V. The other curve depicts the MoS₂ FETdevice 100 characteristics when both split-gates 106 are simultaneouslyswept from 0V to −30V. Unlike the fully top-gated case, I_(DS) isrelatively high when one of the split-gate 106 is at −30V. In otherwords, the MoS₂ FET device 100 cannot be turned OFF by one split-gate106 and can be considered as always ON. However, when both split-gates106 reach −30V simultaneously, the MoS₂ FET device 100 reaches OFF statewith current ON/OFF ratio of ˜10⁶. Furthermore, the MoS₂ FET device 100can be considered as a two-input-one-output digital element representedby the truth table shown in FIG. 10B, which can be identified as NANDlogic. FIG. 11 shows that the output current from the MoS₂ FET device100 is completely suppressed only when V_(SG1) and V_(SG2), of magnitude−30 V, arrive at the two corresponding spilt-gates 106, concurrently.FIG. 11 confirms that a split-gated MoS₂ FET device 100 can be used as acoincidence detector neuron.

n order to mimic the computational map constructed by the coincidenceneurons in the auditory cortex of barn owl, the inventors fabricated abiomimetic audiomorphic device 104 structure shown in FIG. 15 . FIG. 15shows a FET device 100 with a plurality of split-gates 106. Forinstance, the FET device 100 can include a first split-gate 106, asecond split-gate 106, a third split-gate 106, a fourth split-gate 106,and a fifth split-gate 106, each split-gate 106 having a first gate 106a and a second gate 106 b architecture described above. The use of fivesplit-gates 106 is exemplary, and it is understood that any number ofsplit-gates 106 can be used. In an exemplary embodiment, the fivesplit-gates 106 are distributed along a length of the channel 108,separated by spacing d. For instance, the spacing between the firstsplit-gate 106 and the second split-gate 106 can be d₁. The spacingbetween the second split-gate 106 and the third split-gate 106 can bed₂. The spacing between the third split-gate 106 and the fourthsplit-gate 106 can be d₃. The spacing between the fourth split-gate 106and the fifth split-gate 106 can be d₄. Any of d₁, d₂, d₃, d₄, can beequal to another or different. It is contemplated for each of d₁, d₂,d₃, d₄ to be equal to each other so as to have consistent or constantlateral spacing between each split-gate 106. The distance between thefirst gate 106 a and the second gate 106 b of each split-gate 106 can bes. Thus, the first split-gate 106 can have s₁, the second split-gate canhave s₂, the third split-gate can have s₃, the fourth split-gate canhave s₄, and the fifth split-gate can have s₅. Each of s₁, s₂, s₃, s₄,s₅ should be different from each other. It is contemplated for each ofs₅>s₄, s₄>s₃, s₃>s₂, s₂>s₁ so as to monotonically increase s of eachsplit-gate 106 from the source 110 to the drain 112. In an exemplaryembodiment, each of d₁, d₂, d₃, d₄=400 nm, and s₁=200 nm, s₂=300 nm,s₃=400 nm, s₄=500 nm, and s₅=600 nm.

FIG. 16 shows the transfer characteristics of the device when each ofthe five split-gates 106 are concurrently swept from 0V to −30V. Notethat a split-gate 106 can, in principle, be constructed by combining anyone of the first gates 106 a with any other second gate 106 b within the10-gate structure. In fact there exists N(N+1)/2 distinct pairs ofsplit-gates 106 in total which in the exemplary case is 15, since N=5. Aparameter called inhibition ratio (IR) is defined as follows:

$\begin{matrix}{{IR} = \frac{I_{DS}\left( {V_{SG} = {0\mspace{14mu} V}} \right)}{1_{DS}\left( {V_{SG} = {{- 3}0\mspace{14mu} V}} \right)}} & \lbrack 1\rbrack\end{matrix}$

IR is the ratio of current through the FET device 100 corresponding toV_(SG)=0V and V_(SG)=−30V, concurrently applied to any given split-gate106. FIG. 17 shows the IR color map for all possible combinations of thesplit-gates 106. Noticeably the IRs are significantly larger forsplit-gate pairs that are vertically aligned (diagonal elements in thecolor map). Being vertically aligned means that the first gate 106 a andthe second gate 106 b are of the same split-gate 106. It is contemplatedfor these pairs to be used as individual coincidence detector neurons inthe biomimetic audiomorphic device 104. Furthermore, within thesevertically aligned pairs, the one with minimum split-gate spacing, s,has the maximum IR and vice versa. From the map, it is clear that the IRdrops almost exponentially as a function of the split-gate spacing, s.This is interesting since the IR is mostly determined by the currentthat flows through the ungated region at V_(SG)=−30V. Note that at thisV_(SG), the gated regions are switched OFF. Although the electrostaticpotential of the ungated region due to the split-gate potentialdiminishes linearly with the split-gate spacing, the exponentialdependence in IR can be explained from the fact that the device isbiased close to the subthreshold regime, where the device current is anexponential function of the channel potential. Note that the IR colormap can be used to identify the split-gate pair where the coincidencetook place, or in other words the device constructs a spatial map forthe coincidence detection.

FIG. 20 shows the output current from the FET device 100 when randomvoltage spikes of magnitude −30 V are applied to two spilt-gates 106corresponding to different spatial pairs. All pairs detect coincidenceby suppressing the device current. Furthermore, the suppression ismaximum with an IR of ˜10⁵ when coincidence occurred in the split-gatepair corresponding to spacing, s=200 nm, and minimum with an IR of ˜450for spacing, s=600 nm. Clearly, individual split-gate pairs performdigital computation for coincidence detection (NAND logic), while, theFET device 100 as a whole uses analog IR values for determining thespatial location of the coincidence. As such, embodiments of thebiomimetic audiomorphic device 104 using this FET device 100 canseamlessly combine digital and analog computation—a feature that isabundant in biological neural networks. It should be further noted thatthe number of split-gate pairs or coincidence neurons, N_(P), determinesthe number of analog computation levels which in turn determines theangular precision, Δθ, of the biomimetic audiomorphic device followingthe relationship, Δθ=180°/N_(P).

From this expression, it may appear that the angular precision can bemade arbitrarily small by increasing the number of analog levels.However, a large number of analog levels would require a wider analogrange for the current suppression or IR, or in other words itnecessitates a semiconducting channel with large current ON/OFF ratio.If the minimum required difference between the analog IR valuescorresponding to two consecutive split-gate pairs is ΔIR_(D), then N_(P)and hence Δθ will be determined by the following expression:

$\begin{matrix}{{N_{P} = \frac{I_{ON}/I_{OFF}}{\Delta\;{IR}_{D}}};{{\Delta\theta} = {180^{0}\frac{\Delta\;{IR}_{D}}{I_{ON}/I_{OFF}}}}} & \lbrack 2\rbrack\end{matrix}$

FIG. 19 shows Δθ and N_(P) as a function of ΔIR_(D) for different valuesof I_(ON)/I_(OFF). Note that a small value for ΔIR_(D) results ingreater precision but also requires larger number of split-gate pairs.Furthermore, small ΔIR_(D) can be more susceptible to noise.Interestingly, the same precision can be achieved for a larger ΔIR_(D)by increasing the ON/OFF ratio. Nevertheless, the biomimeticaudiomorphic device 104 can be designed to offer orders of magnitudebetter precision than the barn owl. The biomimetic audiomorphic device104 also offers advantages in device footprint and scalability.

As described in the Jeffress model, the auditory cortex exploits thefinite axonal conduction velocity for transforming the binaural acousticinformation encoded via ITDs into a spatial computational map for soundlocalization. However, unlike the solid state electronic circuits wherethe signal propagation through metallic interconnects occur at a veryhigh speed, axonal signal propagation is significantly slower. This isbecause the propagation of action potential along the length of an axonrequires periodic charging of the leaky axoplasomic membrane. Invertebrate axons, the myelin sheath improves the axonal insulation andaction potentials are regenerated only at the nodes of Ranvier throughhigh density sodium ion-channels.

FIG. 21 shows the schematic of an artificial time delay neuron realizedby connecting the gate 106 of a first FET device 100 to a drain of asecond FET device. The first FET device 100 is a split-gated FET device(FIG. 9 ) and the second FET device is a fully top-gated FET device(FIG. 7 ). This facilitates constructing a simple RC circuit 102 by thegate capacitance (C_(G)) and channel resistance (R_(CH)) of thecorresponding FET device.

FIG. 22 shows the experimental transient responses of the artificialtime delay neuron, which can be captured using equation 3:

$\begin{matrix}{V_{OUT} = {{V_{IN}\left\lbrack {1 - {\exp\left( {- \frac{t}{R_{CH}C_{G}}} \right)}} \right\rbrack} = {V_{IN}\left\lbrack {1 - {\exp\left( {- \frac{t}{\tau_{C}}} \right)}} \right\rbrack}}} & \lbrack 3\rbrack\end{matrix}$

In FIG. 22 , the different transient responses correspond to differenttime constants or delays (τ_(C)) achieved by biasing the resistive FETdevice (the fully top-gated FET device) in different regimes ofoperations as shown FIG. 23 . The time constant was found to vary from˜800 μs at V_(TG)=−15V to ˜200 ms at V_(TG)=−30V corresponding to R_(CH)values varying from 4.4 MΩ to 0.9 GΩ, respectively. The gate capacitancedue to large gate metal pads and other parasitic contributions was180±20 pF. The time constant can be reduced by reducing the R_(CH) bybiasing the FET device further into the ON-state.

FIG. 24 shows an embodiment of the biomimetic audiomorphic device 104using embodiments of the FET device 100 and RC circuit architectures.The biomimetic audiomorphic device 104 includes a FET device 100 (havinga split gate architecture) in which each split-gate 106 is connected tothe drain of a corresponding fully top-gated FET device. For instance,the first split-gate 106 is connected to the drain of a first fullytop-gated FET device (having a source₁, drain₁, and gate₁), the secondsplit-gate 106 is connected to the drain of a second fully top-gated FET(having a source₂, drain₂, and gate₂), the third split-gate 106 isconnected to the drain of a third fully top-gated FET device (having asource₃, drain₃, and gate₃), the fourth split-gate 106 is connected tothe drain of a fourth fully top-gated FET device (having a source₄,drain₄, and gate₄), and the fifth split-gate 106 is connected to thedrain of a fifth fully top-gated FET device (having a source₅, drain₅,and gate₅). Thus, the first split-gate 106 can have its first gate 106 aand its second gate 106 b connected to drain₅ and drain₁ of the firstfully top-gated FET device on either side, respectively. The secondsplit-gate 106 can have its first gate 106 a and its second gate 106 bconnected to drain₄ and drain₂ of the second fully top-gated FET deviceon either side, respectively. The third split-gate 106 can have itsfirst gate 106 a and its second gate 106 b connected to drain₃ anddrain₃ of the third fully top-gated FET device on either side,respectively. The fourth split-gate 106 can have its first gate 106 aand its second gate 106 b connected to drain₄ and drain₂ of the fourthfully top-gated FET device on either side, respectively. The fifthsplit-gate 106 can have its first gate 106 a and its second gate 106 bconnected to drain₅ and drain₁ of the fifth fully top-gated FET deviceon either side, respectively. Each of source₁, source₂, source₃,source₄, source₅ of the first, second, third, fourth, and fifth fullytop-gates FETs can be connected to each other at a node.

Note that the resistances of the fully top-gated FET devices connectedto the bottom halves of the split-gates increase monotonically from leftto right and vice versa for the top halves of the split-gates. Thus, inthe exemplary embodiment in which s₅>s₄, s₄>s₃, s₃>s₂, s₂>s₁, the firstfully top-gated FET device has a resistance of R₁, the second fullytop-gated FET device has a resistance of R₂, the third fully top-gatedFET device has a resistance of R₃, the fourth fully top-gated FET devicehas a resistance of R₄, and the fifth fully top-gated FET device has aresistance of R₅, wherein R₅>R₄, R₄>R₃, R₃>R₂, R₂>R₁ for the top halvesof the split-gates and vice versa for the bottom halves. Thisfacilitates imitating the organization of delay axons in the auditorycortex of the barn owl. The required resistance values can be achievedby designing the length and width of the fully top-gated FET devicesaccordingly.

One of the remarkable features of the brain is its plasticity. Forexample barn owls survive by hunting small rodents in the opencountryside in Europe. However, there is a significant change in theground cover available to its prey species between winter and summer.The barn owl must, therefore, become more precise and alert in soundlocalization in winter than in summer by reversibly changing its brainmorphology. Brain morphology can also change due to adaptation to thelocal environment. For example, the precision in sound localizationdiffers between the barn owls found in the Mediterranean versus Europeor North America. Therefore, it may be desirable to introduceneuroplasticity into the biomimetic audiomorphic device 104.

FIG. 25 shows an exemplary schematic of a FET device 128 with globalback-gating capability. This is achieved by including a back-gatedielectric to form a heavily doped back-gate 120 or back-gate electrode.A top oxide layer 116 is formed in and/or on at least a portion of thesource 110. The top oxide layer 116 a is formed on the top surface andis formed in and/or on at least a portion of the drain 112. In someembodiments, the top oxide layer 116 (e.g., HSQ, Al₂O₃, HfO₂, h-BN)spans a length between the source 110 and the drain 112. The bottomsurface of the substrate 114 includes a bottom oxide layer 116 b. Thebottom oxide layer 116 b can be SiO₂, but other materials can be usedsuch as Al₂O₃, HfO₂, h-BN, etc. In some embodiments, the bottom surfaceof the bottom oxide layer 116 b includes a polycrystalline silicon layer126. The bottom oxide layer 116 b and the polycrystalline layer 126 formthe back-gate 120. In the exemplary FET device 128, the back-gatedielectric is 285 nm of SiO₂ and the back-gate electrode 126 is heavilydoped Si.

FET device 128 is configured to have a split gate architecture similarto the embodiment described in the FIG. 9 example. The split-gate 106includes a first gate 106 a formed between the source 110, the drain112, and a MoS₂ semiconducting material 108 on a first side 122 of theFET device 128. The split-gate 106 includes a second gate 106 b formedbetween the source 110, the drain 112, and a MoS₂ semiconductingmaterial 108 on a second side 124 of the FET device 128. The first gate106 a is physically separated from the second gate 106 b. The top oxidelayer 116 a is be formed in and/or on at least a portion of the source110 and at least a portion of the drain 112 so as to span a lengthbetween the source 110 and the drain 112. The semiconducting region MoS₂108 which is underneath the oxide layer 116 a is the active region ofoperation and lies between source 110 and drain 112. A metallic layer118 is formed on top of the top oxide layer 116 a, wherein the metalliclayer 118 and the top oxide layer 116 a form the split-gate 106. In thisconfiguration, the source 110 may be connected to ground so that inoperation, the drain 112 is biased at a drain-source voltage V_(DS), thefirst gate 106 a is biased at a split-gate voltage V_(SG1), the secondgate 106 b is biased at a split-gate voltage V_(SG2), a drain-sourcecurrent I_(DS) flows through the channel 108, and the back-gate 120 isbiased at a back-gate voltage V_(BG). The source 110 and drain 112voltages are biased at 0 and 1V respectively and the current I_(DS)flows from drain to source. The split gates V_(SG1) 106 a and V_(SG2)106 b correspondingly modulate the drain to source current when biasesare applied. The back-gate voltages (V_(BG)) is used to modulate theconductivity of the channel.

FIG. 26 shows the back-gate 120 transfer characteristics for V_(DS)=1V.The back-gate 120 has full electrostatic control over the entirechannel, which is reflected in the large current ON/OFF ratio of 10⁶.The back-gate 120 threshold voltage was found to be V_(TB)=−12V.

FIG. 27 shows the transfer characteristics of the FET device 128 whenthe split-gate pair with 200 nm spacing is concurrently swept from 0V to−30V under different back-gate 120 (V_(BG)) biases. Clearly, theinhibition ratio (IR) changes significantly and non-monotonically as afunction of V_(BG) as shown in FIG. 28 . This can be explained from thefact that when, V_(BG)>>V_(TB), the FET device 128 is in the deepON-state and hence the current through the ungated region remains higheven for V_(SG)=−30V resulting in low IR. As V_(BG) approaches V_(TB),but still remains above threshold, the current through the device forV_(SG)=0V only reduces linearly. However, now it becomes relativelyeasier for the split-gates to switch the ungated channel region from ONstate to subthreshold and reduce the device current exponentially forV_(SG)=−30V resulting in an increased IR. Once V_(BG) goes below V_(TB),the entire device enters subthreshold operation. Now the current throughthe device for V_(SG)=0V also reduces exponentially resulting in adecrease in IR. Finally, for V_(BG)<<V_(TB), the entire device is in thedeep subthreshold regime with current levels beyond the detection limitsof the measurement apparatus and as such the effect of V_(SG) becomesinconsequential.

FIG. 28 shows the IR as a function of V_(BG) for different split-gatepairs. The non-monotonic trend is observed for all split-gate pairs,however, the V_(BG) value for which a given split-gate pair achievesmaximum inhibition ratio decreases monotonically as the split-gatespacing increases. This is expected since larger split-gate spacingtranslates into weaker split-gate control of the ungated region andhence it requires significant electrostatic aid from the back-gate 120.In other words, the back-gate bias should be close to the back-gatethreshold so that the impact of the split-gate pair is stronger in theungated region. In a dual-gated FET, the coupling between thetop/split-gate and the back-gate is critical in understanding the impactof one on the other.

FIG. 29 shows the back-gate transfer characteristics of a fullytop-gated MoS₂ FET for different top-gate voltages (V_(TG)) atV_(DS)=1V. Clearly, the back-gate threshold voltage, V_(TB), depends onV_(TG), which can be explained form the principle of charge balance,i.e. the inversion charge induced by the top-gate must be compensated bythe back-gate and vice versa.

FIG. 30 shows the extracted IR from the VS model simulation results as afunction of V_(BG) for various split-gate spacing ranging from 50 nm to600 nm. To mimic the experimental conditions, the minimum OFF currentthrough the device was restricted to 50 pA/μm corresponding to theleakage floor of the measurement instrument. Clearly, the simulatedsplit-gate transfer characteristics and the IR plots show remarkablesimilarity with the experimental results. Therefore, it can be concludedthat the back-gate bias can be used to tune the IR by orders ofmagnitude and hence can be used to imitate neuroplasticity.

FIG. 31 shows the color map of IR for all possible split-gate pairs ofour biomimetic device under different back-gate biases. Note that alarge difference in the magnitude of IR among the diagonal elements isdesirable for easier detection of the location of coincidence and hencethe localization of the sound source. Clearly, V_(BG)=14 V provides themaximum IR contrast and hence the corresponding color map can be relatedto the hyper-attentive state of the barn owl. As expected, the IRcontrasts in the color maps diminish monotonically as V_(BG) becomesmore positive i.e. the device is biased in the deep ON-state. Therefore,the IR color maps corresponding to V_(BG)=18 V, 22 V and 26 V can becorrelated to the attentive, wakeful and resting state of the barn owl,respectively.

In the conclusion, the inventors have successfully demonstrated thebiomimicry of the neural computational algorithm in the auditory cortexof barn owl. The inventors developed a biomimetic audiomorphic device104 using multiple split-gates on a single semiconducting MoS₂ channelconnected to source/drain contacts for emulating the spatial map ofcoincidence detector neurons and tunable RC circuits for imitating thetime delay neurons following the Jeffress model of sound localization.One of the unique features of this biomimetic audiomorphic device 104 isthe fact that it seamlessly combines digital and analog computation,which is abundant in biological neural networks. In short, individualsplit-gate pairs perform digital computation using NAND logic fordetermining spiking coincidence, while the biomimetic audiomorphicdevice 104 as a whole uses analog values for the inhibition ratio (IR)to determine the spatial location of the coincidence. Further,artificial time delay neurons realized using the semiconducting channelresistance and gate capacitance of MoS₂ FETs allow biomimicking offinite axonal conduction velocity, which is essential for translatingthe ITDs into a spatial computational map. In addition, globalback-gating capability adds tunable and reversible neuroplasticity tothe biomimetic audiomorphic device 104.

It should be understood that the disclosure of a range of values is adisclosure of every numerical value within that range, including the endpoints. It should also be appreciated that some components, features,and/or configurations may be described in connection with only oneparticular embodiment, but these same components, features, and/orconfigurations can be applied or used with many other embodiments andshould be considered applicable to the other embodiments, unless statedotherwise or unless such a component, feature, and/or configuration istechnically impossible to use with the other embodiment. Thus, thecomponents, features, and/or configurations of the various embodimentscan be combined together in any manner and such combinations areexpressly contemplated and disclosed by this statement.

It will be apparent to those skilled in the art that numerousmodifications and variations of the described examples and embodimentsare possible considering the above teachings of the disclosure. Thedisclosed examples and embodiments are presented for purposes ofillustration only. Other alternate embodiments may include some or allof the features disclosed herein. Therefore, it is the intent to coverall such modifications and alternate embodiments as may come within thetrue scope of this invention, which is to be given the full breadththereof.

It should be understood that modifications to the embodiments disclosedherein can be made to meet a particular set of design criteria.Therefore, while certain exemplary embodiments of the device and methodsof using and making the same disclosed herein have been discussed andillustrated, it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied and practicedwithin the scope of the following claims.

What is claimed is:
 1. A biomimetic audiomorphic device, comprising: asubstrate to serve as a back-gate; an insulator or oxide layer formed onthe substrate to serve as a back-gate dielectric; a semiconductingchannel formed on the back-gate dielectric layer; metallic layers formedin and/or on the semiconducting channel to define a source contact and adrain contact; an insulator or oxide layer to serve as a top-gatedielectric; and a metallic layer formed on the top-gate dielectric todefine a split-gate pair, the split-gate pair comprising a first gateand a second gate; wherein the metallic layer formed on the top-gatedielectric defines a plurality of split-gate pairs.
 2. The device ofclaim 1, wherein: the first gate is formed between the source contactand the drain contact located on a first side of the device; the secondgate is formed between the source contact and the drain contact on asecond side of the device; and the first gate is physically separatedfrom the second gate.
 3. The device of claim 1, wherein the substrate isSi.
 4. The device of claim 1, wherein the back-gate oxide layer isselected from the group consisting of SiO₂, Al₂O₃, HfO₂, or h-BN.
 5. Thedevice of claim 1, wherein the semiconducting channel is an organicsemiconductor material or an inorganic semiconductor material.
 6. Thedevice of claim 1, wherein the metallic layer formed on the top-gatedielectric is a stack of nickel or gold.
 7. The device of claim 1,wherein the top-gate oxide layer is selected from the group consistingof Hydrogen silsesquioxane (HSQ), Al₂O₃, HfO₂, or h-BN.
 8. The device ofclaim 1, wherein, in operation: the back-gate is biased at a voltageV_(BG), the source contact is grounded, and the drain contact is biasedat a drain-to-source voltage V_(DS); and the first split-gate is biasedat a split-gate voltage V_(SG1), the second split-gate is biased at asplit-gate voltage V_(SG2), and a source-to-drain current IDS flowsthrough the semiconducting channel.
 9. The device of claim 1, whereinthe plurality of split-gate pairs includes a first split-gate pair, asecond split-gate pair, a third split-gate pair, a fourth split-gatepair, and a fifth split-gate pair.
 10. The device of claim 9, whereineach split-gate pair is separated by a distance, d, and: the distancebetween the first split-gate pair and the second split-gate pair is d₁;the distance between the second split-gate pair and the third split-gatepair is d₂; the distance between the third split-gate pair and thefourth split-gate pair is d₃; the distance between the fourth split-gatepair and the fifth split-gate pair is d₄; and either d₁, d₂, d₃, d₄ areequal to each other, or d₁, d₂, d₃, d₄ are unequal to each other. 11.The device of claim 9, wherein each first gate and second gate of eachsplit gate pair is separated by a distance, s, and: the distance betweenthe first gate and the second gate of the first split-gate pair is s₁;the distance between the first gate and the second gate of the secondsplit-gate pair is s₂; the distance between the first gate and thesecond gate of the third split-gate pair is s₃; the distance between thefirst gate and the second gate of the fourth split-gate pair is s₄; thedistance between the first gate and the second gate of the fifthsplit-gate pair is s₅; and s₅>s₄, s₄>s₃, s₃>s₂, and s₂>s₁.
 12. Thedevice of claim 1, wherein each first gate and second gate of each splitgate pair is separated by a distance, s, and the s for one split gatepair is different from the s for another split gate pair.
 13. Abiomimetic audiomorphic device, comprising: a substrate to serve as aback-gate; an insulator or oxide layer formed on the substrate to serveas a back-gate dielectric; a semiconducting channel formed on theback-gate dielectric layer; metallic layers formed in and/or on thesemiconducting channel to define a source contact and a drain contact;an insulator or oxide layer to serve as a top-gate dielectric; ametallic layer formed on the top-gate dielectric to define a split-gatepair, the split-gate pair comprising a first gate and a second gate; anda plurality of fullytop-gated field effect transistor (FET) devices orresistors.
 14. A fullytop-gated field effect transistor (FET) device,comprises: a substrate to serve as a back-gate; an insulator or oxidelayer formed on the substrate to serve as a back-gate dielectric; asemiconducting channel formed on the back-gate dielectric layer;metallic layers formed in and/or on the semiconducting channel to definea source contact and a drain contact; an insulator or oxide layer toserve as a top-gate dielectric; a metallic layer formed on the top-gatedielectric to define the top-gate a plurality of fullytop-gated FETdevices; and a biomimetic audiomorphic device, comprising: a substrateto serve as a back-gate; an insulator or oxide layer formed on thesubstrate to serve as a back-gate dielectric; a semiconducting channelformed on the back-gate dielectric layer; metallic layers formed inand/or on the semiconducting channel to define a source contact and adrain contact; an insulator or oxide layer to serve as a top-gatedielectric; and a metallic layer formed on the top-gate dielectric todefine a plurality of split-gate pairs; wherein each individualsplit-gate is connected to the drain contact of an individualfullytop-gated FET device.
 15. The device of claim 14, wherein, when inoperation: the back-gate is biased at a voltage V_(BG), the sourcecontact is grounded, and the drain contact is biased at adrain-to-source voltage V_(DS), the top-gate is biased at a voltageV_(TG), and a source-to-drain current IDS flows through thesemiconducting channel.